Gas turbine engines include rotating compressor and turbine wheels. FIG. 1 shows a typical turbine wheel 1 comprising a disk 2 mounted to a shaft 3 which is known as a tieshaft. The disk 2 and shaft 3 are symmetric about the axial centerline 4 of the engine. The amount of unbalance of the wheel is defined by the radial eccentricity of its mass center-of-gravity relative to its geometric centerline. This unbalance, if not controlled, will cause the wheel and shaft to vibrate when rotating inside the engine. To avoid this situation each wheel is balanced before being mounted in the engine. Balancing is accomplished by mounting the wheel on a balance machine that has two rotating mounts, spinning the wheel, and removing material from the wheel until the unbalance comes within allowable limits.
The mounting of the wheel on the balance machine can take various forms, depending upon the configuration of the wheel being mounted, and on available fixturing. In some cases it may be satisfactory to mount the wheel directly on the balance machine. In other cases it may be necessary to create an assembly, consisting of the wheel and other custom made parts, in order to achieve satisfactory results. Generally, it is desirable to mount the wheel on the balance machine in a manner as similar as practical to the way in which the wheel is mounted in the engine. The more the balance machine mounting configuration differs from the actual engine mounting, the more uncertainty is introduced into the balancing process. Inadequate control over the balancing process can ultimately result in costly engine rejections prompted by unacceptable engine vibration levels.
The best possible configuration is one in which the wheel can be directly mounted on the balance machine utilizing the bearing surfaces that support the wheel in the engine. However, often the bearing surfaces that support the wheel in the engine are not on the wheel, or on any components integral with the wheel. In that case, the wheel must be mounted on the balance machine using alternative cylindrically machined surfaces that already exist on the wheel.
Mounting a wheel on these alternative machined surfaces introduces uncertainty in the measured wheel unbalance. The unbalance signal, fed into the balance machine through its rotating mounts, is sensitive to the locations at which the wheel is supported. When the wheel is mounted at different bearing locations than those in the engine, uncertainty is introduced because the balance machine is not capable of accounting for this difference when processing the unbalance input. Further, inaccurate machining of these alternative surface introduces additional uncertainty.
Additional problems occur when the tieshaft is attached to with the disk. In an engine assembly, the tieshaft acts as a through bolt, clamping together engine components that are stacked over the tieshaft. A loaded assembly is created by installing a nut onto the threaded end of the tieshaft, and tightening the nut against the stacked components, thereby loading the tieshaft in tension and the stacked components in compression. The tensile loading on the tieshaft in the assembly acts to straighten out tieshaft curvature, thereby reducing engine unbalance. However, wheels often must be mounted directly on a dynamic balance machine, without the tieshaft straightening benefit of a loaded assembly.
Referring again to FIG. 1, a typical configuration of this type would have the wheel 1 rotatably supported at alternative bearing locations 5 and 6, leaving the tieshaft 3 unsupported. Although uncertainty is reduced because the number of parts being balanced is minimized, this benefit may be more than offset due to the effect of the alternative mount points 5 and 6, combined with a tail wagging effect from the unsupported tieshaft 3. The tail wagging effect can cause amplification of unbalance loads from tieshaft curvature that is not present in the loaded assembly of the engine. As a result, balancing wheels in this way often results in excessive engine vibration.
One approach to improving the balancing of wheels with integral tieshafts is to create a loaded assembly similar to an engine assembly. One such assembly is illustrated in FIG. 1. The components, that in the engine comprise the compressed stack-up, are here simulated by a long sleeve known as a balance arbor 7. It is then the balance arbor 7 that is compressed against disk 2 upon assembly. On the exterior of arbor 7 are machined surfaces 8 that correspond in size and location to the bearing surfaces that support the wheel in an engine. By utilizing machined surfaces 8 for mounting on the balance machine, uncertainty due to bearing location and machining inaccuracy is thereby minimized. Uncertainty is further reduced because the tieshaft 3 is loaded in tension as in the engine assembly, thus acting to straighten any curvature in the tieshaft. Also, by applying the same tieshaft load as used in the engine assembly, any unbalance created by remaining curvature will be the same on the balance machine as in the engine.
Although an assembly like that shown in FIG. 1 solves the mounting configuration problems associated with mounting the wheel alone on the balance machine, uncertainty is increased by the fact that additional parts are required. Referring again to FIG. 1, the balance arbor 7 is radially positioned relative to the wheel 1 by an overlapping radial fit of the arbor to a machined surface of the disk. Due to a combination of machining inaccuracy, and a need to re-use the balance arbor, there is inevitably some looseness in the assembly resulting in lack of control over the radial positioning of the wheel relative to the arbor. Thus, uncertainty is introduced because the relative radial position of the wheel is not completely controlled, either from one assembly to the next, or from the balance machine to the engine.
Accordingly, a need exists for a balance arbor that can simulate the mounting configuration and loading conditions present in the engine, while minimizing uncertainty in the unbalance measurements due to looseness of the assembly.